The piston sealing device according to patent application FR 3009037 is designed for a piston moving in a cylinder one of whose ends is closed by a fluid chamber, said piston comprising at least one piston head having at least one fixed skirt, said head having on the one hand a piston bearing surface to exert a force on any means of mechanical, hydraulic, or pneumatic transmission and, on the other hand, a compression surface opening into the fluid chamber and able to receive the pressure of a fluid.
Again according to the patent application FR 3009037, the piston sealing device comprises a sliding skirt of cylindrical shape lodged in a chamber with slight gap. This skirt is moreover placed as a prolongation of the piston head on the side with the compression surface and in the axis of said head.
It will be noted that said sliding skirt is connected to said head by a mechanical interskirt connection which allows it to move in longitudinal translation with respect to said head, however a pressure transmission channel devised inside the sliding skirt passes straight through the latter in the axial direction.
The invention of patent application FR 3009037 is likewise characterized by a continuous extensible segment of continuous annular shape which is inserted between the fixed skirt and the sliding skirt. This segment comprises an internal cylindrical segment face subjected to the pressure of the fluid via the pressure transmission channel, an external cylindrical segment face able to make contact with the cylinder, an axial segment face on the fixed skirt side held directly or indirectly in tight contact with the fixed skirt and an axial segment face on the sliding skirt side held directly or indirectly in tight contact with the sliding skirt.
Finally, the invention regarding patent application FR 3009037 is also characterized by a sliding skirt spring which tends to bring the sliding skirt closer to the fixed skirt and to axially compress the continuous extensible segment or at least prevent said sliding skirt from moving away from the fixed skirt in the manner of an end stop.
Upon perusal of patent FR 3009037, one understands that the maximum leakage rate of fluid flowing between the piston and the cylinder occurs when the pressure prevailing in the fluid chamber is high enough to maximize the flow rate of fluid moving in the gap left between the sliding skirt and the cylinder, but not yet high enough for the continuous extensible segment to be pressed in circumferential contact against the cylinder, realizing a nearly total tightness.
Thus, according to the piston sealing device per the patent application FR 3009037, a leakage thus exists in transitory manner between the piston and the cylinder. This leakage is furthermore added to that which occurs during the low-pressure phase found during the delivery stroke of a hydraulic motor or during the aspiration stroke of a hydraulic pump. In fact, during this phase, the continuous extensible segment is parked and does not provide any tightness other than that consisting in the restriction of the movement of fluid formed by the gap left between said segment and the cylinder.
This leakage is necessary to the lubricating and cooling of the continuous extensible segment, on the one hand, and the piston on the other.
Depending on the type of pump or hydraulic motor for which the piston sealing device of patent application FR 3009037 is being used, the need for lubricating and cooling fluid may vary, especially depending on the magnitude of the radial force exerted by the piston on the cylinder.
In fact, the larger this force, the more energy is dissipated by friction and shear of oil at the interface between the piston and the cylinder. In order to carry away this energy, there must be sufficient oil circulating.
Moreover, the need for lubricating oil tends to increase with said force, since there must be guaranteed in all circumstances a sufficient thickness of the oil film separating the piston and the cylinder, so as to prevent any metal on metal contact between said piston and said cylinder.
Furthermore, it will be noted that certain pumps or certain hydraulic motors comprise mechanical parts which are positioned opposite the fluid chamber receiving the fluid under pressure and which are lubricated and/or cooled thanks to the leakage flow between the piston and the cylinder.
The pronounced tightness of the piston sealing device according to patent application FR 3009037 may at times cause a problem of insufficient lubricating and/or cooling rate both for the piston itself and for any mechanical parts with which the piston is cooperating.
One solution might involve placing one or more oil injectors in various locations of the pump or hydraulic motor casing to lubricate and/or cool the components just mentioned. The problem is that such injectors would inevitably complicate the hydraulic pump or the hydraulic motor receiving them. Moreover, the lubrication of the piston in its cylinder may still be inadequate, since the flow rate of oil furnished by such injectors must reach parts of the cylinder made inaccessible by the piston.
The cooling and lubricating system for a piston sealing device according to the invention is thus addressed primarily to the pumps and hydraulic motors outfitted with the piston sealing device according to patent application FR 3009037 whose flow of lubricating and cooling fluid has become inadequate due to the pronounced tightness realized by this device.